Polyester apparel fibers are for the most part produced from a standard homopolymer polyethylene terephthalate base using either dimethyl terephthalate/diethylene glycol or terephthalic acid/diethylene glycol as starting materials. The commercial need to minimize polymer discontinuities during the polymer spinning operation has mostly required that the polymer have a sufficientdegree of polymerization (18 to 24 LRV) to provide a spinnable melt. Polymer viscosities between 800 and 1800 poise have generally been employed, because spinning discontinuities have been more numerous at lower viscosity, een below 1000 poise. Additives such as the branching agents described by Mead and Reese in U.S. Pat. No. 3,335,211 and by Vaginay in U.S. Pat. No. 3,576,773 can be added to the lower chain length materials in order to raise polymer viscosity and allow commercial spinning performance. "By trifunctional or tetrafunctional branching agent" herein is meant trifunctional and tetra functional branching agents of the type disclosed by Mead and Reese and by Vaginay.
Fibers made in the above manners generally have at least one and sometimes two undesirable properties. For instance, fibers produced with high LRV polymer give knit and woven fabrics and garments that form undesirable "pills" from surface abrasion during normal wear and require high energy superatmospheric and/or enviromentally undesirable chemical additives to the dye systems to effect the coloration desired and necessary for commerce. Fibers and fabrics produced from reduced chain length polymers by the incorporation of melt viscosity enhancing additives that may give reduced "pill" fabrics and garments, still require the use of the high energy dye or chemical systems to achieve coloration.
Historically, polymer modifications (e.g. polyethylene oxides, adipates, glutarates, etc.) have been used to enhance the dye rate and styling versatility in carpet and rug fibers of high dpf (typically 6 and above dpf, denier per filament) and allow easy atmospheric dyeing. Dye enhancing polymer modifications have been practically limited to less sensitive non-textile end uses because the modifiers depress polymer viscosity and can lead to excessive spinning discontinuities that render spinning processes non-commercial and fabrics non-saleable because of excessive defects. The normal route to correcting spinning performance has been to increase the degree of polymerization(LRV). This has had the unwanted effect of increasing fiber strength and subsequently fabric pilling.